The startle response will be studied in autistic and normal 3 to 8 year old children in order to obtain quantitative neurophysiologic data on the possible pathophysiology of this important brain stem reflex in autistic children. Clinically, these children appear to have a reduced startle response as part of their generally aberrant reactions to sensory stimulation. Startle reponse components to be recorded include EMG from orbicularis oculi and selected antagonistic flexors and extensors, the blink reflex, change in heart rate, and evoked and background EEG activity. All response components will be recorded quantitatively by a computer system which also will control the time of administration of stimuli according to the prestimulus state of the subject. Additional control over the prestimulus state will be obtained by the use of a warning stimulus which has been found to increase the predictability of responses to startle stimuli. Three experiments are designed to provide data on the relative degree of inhibition and facilitation, and habituation and sensitization, of startle under physiologically defined conditions of orientation or arousal. This proposal is Phase II of proposal originally submitted to NIMH. The proposal to study autistic children was postponed until feasibility of studying startle responses in young normal children could be demonstrate (Phase I). The latter has now been accomplished, and we are ready to pursue the study of startle responses in autistic children, as originally consisdered by NIMH, with modifications based on our most recent experience.